Determination of particle charge may be important in certain problems related to monitoring indoor and outdoor environments, geological and other atmospheric processes, as well as the processing and handling of industrial powders.
Instruments that measure charge on particles have typically employed indirect measurement techniques in which the particle is exposed to an electrical field while the particle motion in response to the field is monitored optically or by other means. Electrical mobility analyzers are an example of an indirect measurement but particles having a size range greater than a few micrometers are not easily handled by electrical mobility analyzers, which require a relatively high electric fields.
Furthermore, electrical mobility analysis generally bases the measurement on the assumption that the particle carries at most one or two electrons of charge in order to provide size information about the particle. Other indirect particle charge measurement devices may include single particle traps and balances. While these may be relatively sensitive, they are not easily automated, and it may be difficult to use these devices to obtain rapid measurements.
The single particle aerodynamic relaxation time (SPART) analyzer is described in U.S. Pat. No. 4,633,714: Aerosol particle charge and size analyzer. This is another indirect approach device which acquires images of tracks of aerosol particles falling through an oscillating electric field, locates pixels in each image that form the individual tracks, and forms non linear curve fits of the tracks in order to determine track parameters. These parameters are used to estimate the size of each particle and the charge on each particle. This device may be physically very large and costly to build.
Other aerosol instruments measure the particle aerodynamic diameter (but not charge) by monitoring the motion of particles due to an accelerating flow. The Aerosizer, made and marketed by TSI, Inc is described in U.S. Pat. Nos. 4,938,592; 4,917,494; and 5,641,919. The Aerosizer instrument particles velocities are determined in the expansion region of a free jet after the particle acceleration is virtually complete. Furthermore, its velocity measurements do not make use of an induced electrical signal from individual particles.
Published research has demonstrated the use of a cylindrical electrode to sense the charge on particles in a carrier gas transported through a relatively large diameter conduit with glass walls (eg. Gajewski, J. B. and A. Szaynok, Journal of Electrostatics, vol. 10, (1981) page 229). However, that work is concerned strictly with the measurement of charge carried by ensembles of particles, not the measurement of charge on individual particles.
The present application teaches a device which may measure the charge and size of a large number of airborne particles. The charge on an individual particle may be detected by using a gas flow to draw a particle through a cylindrical electrode that acts as a Faraday cage.
According to an embodiment, an imposed gas flow inside a dielectric capillary tube may be used to guide the particle through the charge sensing volume. The charge is sensed through the capillary tube.
The gas stream to draw the particle through the cylindrical sensing region may accelerate particles of different sizes accelerate at different rates in the flow. This may allow measurement of particle size via velocity observation.
One use of the present system is in determination of electrostatic charge on individual particles or droplets suspended in a gas. The system may also be used to simultaneously determine the aerodynamic diameter of each charged particle that is measured.